And Conservative sources say there is no new evidence to suggest that police need more than 28 days.

But Constitutional affairs minister Harriet Harman, one of six Labour MPs vying to be Mr Brown's deputy, said she thought MPs would back new laws - including 90 day detention - if Mr Brown could prove they were needed.

"I don't think there will be a huge problem if there is a proper debate about it - if evidence is brought forward about why current powers are inadequate and what the safeguards will be," she told BBC One's Sunday AM.

In a speech on Saturday, Mr Brown said stronger measures would have to be put in place to give the authorities the power to intervene at earlier stages of an investigation.

"That's why we will need to strengthen the policing resources available. But at every stage I would say this.

"Because we are a country that believes in civil liberties of the individual, every time you have to strengthen the security measures that are necessary to protect our country, you also have to strengthen the accountability to parliament and the independent oversight of what police and other authorities are doing."

'Grave mistake'

Shadow Home Secretary David Davis, for the Conservatives, criticised the timing of Mr Brown's announcement.

He said: "It is extraordinary that the chancellor has chosen to publicise these proposals five days before the home secretary announces his counter-terrorism plans in Parliament.

"It does not auger well for cross-party attempts to build a consensus for counter-terrorism measures, which the whole country needs to get behind."

Shami Chakrabati, director of pressure group Liberty, welcomed the phone intercept proposals but said Mr Brown was making "a grave mistake" in proposing to extend questioning without charge beyond 28 days.

"Twenty-eight days is already the longest period to hold a person without charge in the free world. If you go beyond 28 days it is internment," she told BBC News.